When is a heat wave more than a heat wave? When it’s actually a heat dome.

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The National Oceanic and Atmospheric Administration (NOAA) said today that dangerous heat is expected to grip much of the central and northeastern United States ahead of the Fourth of July weekend. Temperatures are expected to climb well into the 90s from the lowerGreat Lakes region to New England, according to NOAA, with record highs in a number of states—including New York, Pennsylvania, and New Jersey.

Indeed, it looks as though few states will be spared. NOAA’s maximum heat index map for Monday, July 6, has more shades of red than an Italian winery. If you’re looking to escape the heat, NOAA suggests making a break for coastal Southern California, the Pacific Northwest, or the inland across the Northern Rockies.

[NOAA]The Weather Channel is blaming an “upper-level ridge of high pressure,” which is sometimes called a “heat dome.” I confess I had to Google this term to learn what it meant. Apparently, it’s been in use for a few years—at least by the Weather Channel. A New York Times article referenced the term “heat dome” back in 2016, attributing it to “weather people,” which is another way of saying the Weather Channel.

That same year, NBC News offered a mini explainer of the term, courtesy of meteorologist Sherri Pugh. Here’s how she described it:

“You have a building ridge south of the jet. That area acts as a dome, and then under this dome, you have a lot of sinking air. Sinking air is compressed and therefore it warms, and that adds an excessive amount of heat in the dome.”

I also traced the origins of the #HeatDome hashtag on Twitter. The earliest references I could find came in 2011. The Twitter account for NBC’s Today show used the hashtag to accompany one if its videos.

So how does “heat dome” differ from “heat wave”? According to NOAA, a heat wave generally applies to unusually hot weather that lasts for a period of two days or more. For it to be considered a heat wave, temperatures must be outside the normal range—like 95 degrees in New England, for instance. But waves and domes are not always indistinguishable, as NOAA says, “Heat waves are generally the result of trapped air.” As in, trapped under a dome!

Funny weather names aside, extreme heat is no laughing matter. The National Weather Service has released some important tips on what to do to stay safe when the mercury rises. Read more here or via the embed below.

A sweltering Heat Wave is consuming the middle of the country with widespread Excessive Heat Warnings and Advisories. These warnings/advisories will spread into the East for the weekend. It’s worth a moment to keep these tips in mind during this likely and lengthy event! pic.twitter.com/gZ0gqlqNyN